The demand for high power lasers in different wavelengths is growing as new applications are found and the number of gain media in different wavelength regions remains limited. The primary mechanism used to change laser wavelengths to different spectral regions is the use of nonlinear crystals to convert the fundamental laser wavelength to new wavelengths. Examples of such systems include the conversion of the wavelength at 1064 nm from Nd:YAG lasers to a wavelength of 532 nm using nonlinear crystals such as Potassium Dihydrogen Phosphate (KDP), Barium Borate (BBO), Lithium Triborate (LBO), Bismuth Borate (BiBO), and Potassium Titanyl Phosphate (KTP). This light at 532 nm can be further converted to 355 nm by summing the resultant 532 nm radiation with the remaining laser fundamental at 1064 nm in another crystal to generate 355 nm. The 532 nm can also be converted to 266 nm by doubling in crystals such as BBO, Cesium Dihydrogenarsenate (CDA), Potassium Fluoroboratoberyllate (KBBF) and Cesium Lithium Borate (CLBO). The 266 nm can be converted by summing with the fundamental at 1064 nm to get to wavelengths as short as 213 nm.
In all of these processes, it is theoretically possible to attain conversion efficiencies of the fundamental laser wavelength to the desired wavelength range by as high as 100% for flat top spatial/temporal laser pulses. In practice, conversion efficiencies as high as 80-90% for second harmonic generation (SHG) and 30-40% for third harmonic generation (THG) to ultraviolet (UV) are attained using spatial-temporal shaped pulses and/or effective multi-pass operation of the nonlinear crystals.
The above wavelengths have found an extremely wide array of applications from laser machining (e.g., marking/engraving/cutting) of materials, to range finding, to laser surgery/dental applications. The different wavelengths have specific attributes related to the absorptivity of the light in the medium of interest that reflect the importance in changing wavelengths. In some cases, the desired wavelength is for increased transparency, such as in the case of range finding. In other cases, it is for higher absorption, as in the case of highly confining the energy for cutting materials through superheating and ablation. In all cases, the desired application benefits from higher laser power and frequency conversion to higher powers in the wavelength of interest as higher power either enables larger signals or faster processing.
Solid state lasers have been scaled to increasingly higher powers to attain significant brightness. For example, it is now possible now to produce continuous (CW) lasers with diffraction limited output at 1 KW, which corresponds to a focusable average power of more than 1011 W/cm2. Despite the high power scaling capabilities of the fundamental laser source, it has not been possible to scale the nonlinear frequency conversion to take full advantage of the higher power inputs. Thus, there is a need in the art for methods and systems for reducing surface damage in nonlinear crystals used for efficient, high power frequency conversion of laser light.